1. Field of the Invention
The present invention relates to a method for producing a Group III nitride-based compound semiconductor through the flux process, in which nitrogen (N) is reacted with a Group III element of gallium (Ga), aluminum (Al), or indium (In) in a flux mixture containing an alkali metal, to thereby grow a Group III nitride-based compound semiconductor crystal. As used herein, “Group III nitride-based compound semiconductor” encompasses a semiconductor represented by the formula AlxGayIn1-x-yN (0≦x≦1, 0≦y≦1, 0≦x+y≦1); such a semiconductor containing a predetermined element so as to attain, for example, an n-type/p-type conduction; and such a semiconductor in which the Group III element has been partially substituted by B or Tl, and the group V element has been partially substituted by P, As, Sb, or Bi.
2. Background Art
There have been developed techniques for precipitating gallium nitride (GaN) by exposing nitrogen or ammonia to a melt (flux mixture) of gallium (Ga) and sodium (Na) under pressurized conditions. In such a technique, when a seed crystal or a substrate is provided in such a flux mixture, gallium nitride (GaN) is deposited on a surface of the seed crystal or substrate. Thus, a gallium nitride (GaN) single crystal having a thickness of several millimeters can be produced.
Hitherto reported techniques employing a substrate provided in such a flux mixture include a technique in which a gallium nitride (GaN) single crystal is grown on a surface of a substrate made of a material different from a Group III nitride-based compound semiconductor (hereinafter the substrate may be referred to as a “hetero-substrate”) (Japanese Patent Application Laid-Open (kokai) No. 2005-187317); and a technique in which a gallium nitride (GaN) film is epitaxially grown on a hetero-substrate to form a template, and a gallium nitride (GaN) single crystal is grown on the gallium nitride (GaN) film (Japanese Patent Application Laid-Open (kokai) No. 2005-194146). However, those techniques pose a problem in that when a GaN single crystal is grown on a hetero-substrate in the flux mixture at high temperature and high pressure, followed by cooling to room temperature, cracks are likely to be generated in the thus-grown single crystal, since the hetero-substrate has a lattice constant and expansion coefficient different from those of a Group III nitride-based compound semiconductor. In addition, in a technique employing a template formed by growing a gallium nitride (GaN) film or the like on a hetero-substrate, the temperature of the template is lowered from high epitaxial growth temperature to room temperature during formation thereof, and a GaN single crystal is grown on the template in the flux mixture at high temperature and high pressure, followed by cooling to room temperature. Therefore, cracks are more likely to be generated in the thus-grown single crystal. In consideration of such problems, it is desirable to use a self-standing substrate having the same composition as a Group III nitride-based compound semiconductor which is to be produced.
When a so-called GaN self-standing substrate having a c-plane main surface is employed as a seed crystal, crystals are deposited on both surfaces (i.e., gallium-face (Ga-face) of the front surface and nitrogen-face (N-face) of the back surface) of the substrate. Although a single crystal is grown on the Ga-face, three-dimensional crystal growth tends to occur on the N-face, and smooth crystal growth is difficult to attain on the N-face. Therefore, a GaN single crystal grown on the N-face exhibits poor quality, and thus is problematic when provided as a commercial product. In other words, the N-face is a surface which wastes a raw material.